The repository contains several files and branches and I already made
some merges, so I would expect that at least something is displayed.
The tool itself works as it prints a complete graph when called with

I haven't found any documentation about svn-graph.pl, so I also don't
know exactly what this tool is supposed to do. If anyone has a idea what
could be wrong or know a documentation about svn-graph.pl, I would be
glad if (s)he could help me out.